The present invention relates to a corona charging device for depositing charge on an adjacent surface. More particularly, it is directed to a corona charging arrangement usable in a xerographic reproduction system for generating a flow of ions onto an adjacent imaging surface for altering or changing the electrostatic charge thereon.
In the electrophotographic reproducing arts, it is necessary to deposit a uniform electrostatic charge on an imaging surface, which charge is subsequently selectively dissipated by exposure to an information containing optical image to form an electrostatic latent image. The electrostatic latent image may then be developed and the developed image transferred to a support surface to form a final copy of the original document.
In addition to precharging the imaging surface of a xerographic system prior to exposure, corona devices are used to perform a variety of other functions in the xerographic process. For example, corona devices aid in the transfer of an electrostatic toner image from a reusable photoreceptor to a transfer member, the tacking and detacking of paper to the imaging member, the conditioning of the imaging surface prior, during and after deposition of toner thereon to improve the quality of the xerographic copy produced thereby. Both d.c. and a.c. type corona devices are used to perform many of the above functions.
The conventional form of corona discharge device for use in reproduction systems of the above type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating d.c. voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternatively, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,295 wherein an a.c. corona generating potential is applied to the conductive wire electrode and a d.c. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the surface to be charged. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
A problem associated with conventional corona discharge devices employing a conductive wire is a result of the fact that corona glow is associated with a region of high chemical reactivity where chemical compounds are synthesized from machine air, which results in chemical growths being built up on the surface of the wire. These chemical growths after a prolong period of operation degrade the performance of the corona device. Since free oxygen and ozone are produced in the corona region the corona electrode must of necessity be highly oxidation resistant. The above problem of chemical growth build-up on the wire has been addressed by the provision of wire materials which are less subject to chemical attack. While this has reduced the problem, such materials have substantially increased the cost of corona devices.
A still further problem associated with corona discharge devices operating in a xerographic environment results from toner accumulation on the surface of the corona electrode. The spots of accumulated toner, being dielectric in nature, tend to cause localized charge build up on the interior surfaces of the shield which produces current nonuniformity and reduction in corona current. Localized toner accumulations on the insulating end blocks which support the wire electrode also cause sparking. A corona charging device which alleviates the above-noted problems has been disclosed in application Ser. No. 651,769 in the joint names of D. Sarid and B. Springett entitled Compact Corona Charging Device filed concurrently herewith. The invention is directed to a new arrangement incorporating as a part thereof several charging devices constructed in accordance with the above application.
Notwithstanding, the improved results obtained by employing a corona device according to the aforementioned application, the latter two problems, degradation of the coronode surface and contamination of the shield may still require that the corona device be periodically cleaned and if necessary, replaced. The process of replacing corona devices is, of course, an expensive and time consuming procedure which requires the presence of a trained technician. Normally, the worn wire electrode is removed from the corona device and replaced by a new wire. Alternatively, the entire unit is removed, with the shield being cleaned with an abrasive material to remove dirt and toner accumulations.
It would, therefore, be desireable to facilitate replacement of contaminated corona devices in electrophotographic machines with new ones.